<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
           margin: 0;
           padding: 0;
        }
        body,html{
            width: 100%;
            height: 100%;
        }
        .main{
            box-sizing: border-box;
            padding: 40px;
            width: 100%;
            height: 100%;
            background-color: skyblue;
            overflow: hidden;
        }
        .box{
            position: fixed;
            width: 100%;
            bottom: 0;
            left: 0;
            background-color: red;
        }
    </style>
</head>
<body>
    <div class="main">
        <div class="box">dfjsaijkl</div>
    </div>
</body>
</html>